Can't help you with the split/3 under difference, as I only shoot 3 under, however, when I start missing high, I look for one of these things:
1. My string elbow starts drooping. The line of force is along my string forearm, so when it droops, it sends the arrows high.
2. My anchor shifts lower, sending the arrows high. A remarkably small shift in anchor will do this, as little as 1/8".
3. I peek. Until someone pointed this out to me, I would have sworn I kept my head still, but I was moving it and just not aware of the movement.
4. I pluck. Normally, this will throw the arrows to the right, but there is a particular type of upwards pluck that I haven't really figured out completely that throws the arrows up. In any event, the solution is the same: maintain back tension until after the shot is fired.